1. Field of the Invention
The present invention relates to articulated arm transfer devices, such as Cluster Tool Robots, and more particularly to a method and means for shortening the transfer time during robot operation with frictionally held substrates.
2. Problem to be Solved
Material handling robots, e.g., Cluster Tool Robots, are commonly used in the atmosphere or vacuum chambers for handling workpieces such as semiconductor wafers, flatpanel displays, and other like substrates, during processing. These tools consist of a rotatably and transferably mounted plurality of extendible and retractable arms, each having an end effector for supporting a workpiece during arm manipulation. Some examples of such devices are disclosed in U.S. Pat. Nos. 4,666,366, 4,730,976, 4,909,701, and 5,180,276.
Unlike conventional Industrial Robots which provide means for securing payloads when held and manipulated, substrate handling robots, such as Cluster Tool Robots, typically rely on the friction between the load, i.e., the substrate, and the end effector to hold the substrate thereon. This is due to the fact that securing of the payload with mechanical clamping on the top side or edges usually results in objectionable particulate generation and contamination or in substrate damage. The use of a vacuum holddown for securing the payload in atmospheric robots avoids these problems but other control problems can arise if acceleration is changed instantaneously as will be discussed below. The problem in frictional hold applications is that the substrate will slide if the acceleration force at the load exceeds the frictional force holding it, so that the operating or transfer time of the robot arms in their working trajectory is limited by the magnitude of this frictional force. Accordingly, when attempting to increase substrate handling throughput by shortening the transfer time of the robot, a problem is presented in evaluating how the speed of the robot can be increased or optimized, during the multiple motions necessary to move material through a process tool, without causing sliding of the substrate on the end effector.
With most existing substrate handling robots, a method known as "trapezoidal velocity profile" is used to profile their motion. Two problems with this method are:
1) the acceleration rate and velocity is fixed throughout the motion, so that they must be lowered to prevent any one point in the profile from violating a constraint, regardless of the fact that the rest of the profile is not optimal; and
2) sudden changes in the acceleration (transitional points in the velocity) excite vibrations in the system which limit acceleration since, when a substrate is being carried, the command velocity and acceleration must be lowered to account for the added acceleration due to vibration, and when a substrate is not being carried, the commanded acceleration must be lowered to account for the wasted torque used to control the vibrations.
A modified form of the "trapezoidal" approach is the "S-curve trajectory" approach that controls the sudden change in acceleration and, in turn, reduces the vibration. However, since the "S-curve" approach, like the "trapezoidal" approach, does not maintain optimal velocity throughout the motion, ultimately time is lost.
An illustration of the "trapezoidal velocity profile" and the "S-curve profile" as compared to an optimal profile for minimizing transfer time is seen in FIG. 1, which shows velocity with respect to time curves for commanded velocity at the robot arm drive motor. Corresponding acceleration with respect to time curves are shown in FIG. 2 for acceleration at the substrate. It will be seen that at the transitional points, i.e., at the velocity slope change, excessive vibration can occur with the "trapezoidal" approach due to abrupt acceleration change. These vibrations can disturb the substrate position on the end effector causing substrate placement errors and substrate damage in certain process tools. The closed form trapezoidal, i.e., "S-curve", algorithm generates trajectories substantially without the transition effects and the resulting vibration, but, it will be noted that the S-curve trajectory results in an acceleration sag at the transitional points which causes a loss in velocity and a sub-optimal total transfer time from start at the beginning end point to stop at the final end point.
An algorithmic approach that can be used to find the time optimal trajectory for torque limited Industrial Robots is known and described in the following series of IEEE papers, incorporated herein by reference,:
1) K. G. SHIN ET AL, "Minimum-Time Trajectory Planning for Industrial Robots with General Torque Constraints", IEEE Transactions on Automatic Control, vol.AC-31, No.6, June 1986, pps.412-417;
2) F. PFEIFFER ET AL, "A Concept for Manipulator Trajectory Planning", IEEE Journal of Robotics and Automation, Vol. RA-3, No.3, April 1987, pps.115-123;
3) J-J. E. SLOTINE ET AL, "Improving the Efficiency of Time-Optimal Path-Following Algorithms", IEEE Transactions on Robotics and Automation, Vol.5, No.1, February 1989, pps.118-124; and
4) Z. SHILLER ET AL, "Robust Computation of Path Constrained Time Optimal Motions", IEEE Transactions on Robotics and Automation, 1990, pps.144-149.
The application of this algorithmic approach in unmodified form, however, can cause the excitation of resonances in the robot arm when changing command acceleration suddenly, and it does not constrain the acceleration of the payload or substrate so that, if applied to substrate handling robots, sliding of the substrate may occur. The components of the acceleration occurring at the substrate can be broken into this excitation of uncontrolled resonances and the commanded acceleration, but the algorithm offers no suggestion of how to control the resonances to avoid the sliding.
Objects:
It is therefore an object of the present invention to provide a method and means for increasing the throughput of substrate handling tools, such as Cluster Tool Robots, by shortening the transfer time of the robot.
It is another object of the invention to provide a method and means for producing a time optimal robot arm trajectory for increasing substrate handling tool throughput.
It is a further object of the invention to provide a method and means for producing a time optimal robot arm trajectory for increasing substrate handling tool throughput which avoids sliding of the payload or substrate with respect to its support.
It is also an object to provide a method and means that minimize vibration in the robot arm and thus allow time to be minimized with or without a substrate.